During the year in progress new findings on the metabolic activity of myelin and effect of diabetes on these have merged. The incorporation of DL-(1-C14) leucine into individual polypeptides of control and diabetic rats was studied using polyacrylamide gel electrophoresis. The major polypeptide component of rat sciatic nerve (molecular weight 28,000; 58.5% of mass of proteins) was not labelled in either control or diabetic rat. In diabetes incorporation rate into a polypeptide of molecular weight 23,000 which constitutes 21% of total mass was approximately 1/2 that of controls. In polypeptides of molecular weight 38,000 to 49,000 that are heavily labelled in normal animals but constitute only about 5% of total mass of proteins, depression of incorporation was even more marked in diabetes. These findings have been accepted for publication and will appear in the May 1975 issue of J. Clinical Investigation. In non-diabetic animals, the in vitro addition of insulin (10 to the minus 7th power M) stimulated incorporation of DL-(1-C14) leucine into myelin proteins 1.6 to 3.1 times that of controls. This stimulation by insulin in vitro was not seen in diabetic animals. A protein kinase associated with highly purified preparations of myelin has been detected. When purified myelin is incubated with ( - P32) ATP, specific myelin proteins are phosphorylated. This reaction is shown to be Mg2 ions dependent.